This invention relates to the production of gratings on the outer surfaces of optical waveguides. Waveguides, particularly but not exclusively optical fibres, with gratings can be used to make various useful optical devices such as frequency selectors for semiconductor lasers.
In recent years it has been widely reported that spontaneous periodic structures or ripples have been observed after the illumination of metals, semiconductors, and dielectrics by intense laser pulses. This is due to one or more Fourier components of a random surface disturbance scattering light from an incident beam very nearly along the surface. The interference of this diffracted optical wave with the incident beam gives rise to optical interference fringes which can reinforce the original disturbance. Sinusoidal corrugations on either metallic or molten surfaces are found to provide positive feedback for ripple growth.
An object of the invention is to provide a method, using the above phenomenon to produce gratings on waveguides.